Foamed polymeric materials are well known, and typically are produced by introducing a physical blowing agent into a molten polymeric stream, mixing the blowing agent with the polymer, and extruding the mixture into the atmosphere while shaping the mixture. Exposure to atmospheric conditions causes the blowing agent to gasify, thereby forming cells in the polymer. Under some conditions the cells can be made to remain isolated, and a closed-cell foamed material results. Under other, typically more violent foaming conditions, the cells rupture or become interconnected and an open-cell material results. As an alternative to a physical blowing agent, a chemical blowing agent can be used which undergoes chemical decomposition in the polymer material causing formation of a gas.
Foamed polyolefins are known. Of these, polyethylene is preferred because of ease of foaming control. While foams including polypropylene components are known, in most cases such foams include significant proportion of additives that add controlability to the foaming process.
U.S. Pat. No. 4,940,736 (Alteeping) describes a foamed product made by foaming a composition including a major proportion of a low viscosity polypropylene having a melt viscosity of less than 2.times.10.sup.3 poise and a minor proportion of a high viscosity polypropylene having a melt viscosity of greater than 2.5.times.10.sup.3 poise. Alteeping mentions that previously-proposed procedures for foaming polypropylene had suffered from severe disadvantages limiting their commercial application, noting specifically the following: U.S. Pat. No. 4,352,892 (Firma Carl Freudenberg), which discloses foaming a composition including crystalline polypropylene and a further component selected from polybutadiene, ethylene vinyl acetate copolymer, and ethylene-propylene terpolymer rubbers; U.S. Pat. No. 4,442,232 (Firma Carl Freudenberg) which discloses foams comprising crystalline polypropylene and polybutadiene that are cross-linked; U.S. Pat. No. 4,298,706 (Karengafuchi Dagaku Koguyo KK) which discloses foams of compositions comprising of polypropylene and polybutadiene kneaded together; U.S. Pat. No. 3,846,349 (Sumitomo Chemical Co.) which describes foam produced from a three-component mixture of crystalline polypropylene, non-crystalline propylene, and low density polyethylene; and U.S. Pat. No. 3,607,796 (Grunzweig and Hartmann AG) which describes a process for producing foam from a composition comprising high and low molecular weight polypropylene.
U.S. Pat. No. 5,180,751 (Park) describe polypropylene foam made of polypropylene resins having a z-average molecular weight above 1.times.10.sup.6 and a z-average molecular weight/weight average molecular weight ratio above 3.0. Park states that unacceptable foam sheets show a unimodal molecular weight distribution, while resins which yield acceptable foam sheets show a bimodal molecular weight distribution.
U.S. Pat. No. 4,832,770 (Nojiri) describes a method of manufacturing a foamed polypropylene resin from a mixture of 80 to 20 weight percent of a crystalline polypropylene-ethylene block copolymer containing 20 weight percent or less of ethylene and having a melt index of two or less and 20 to 80 weight percent of a crystalline polypropylene-ethylene block or random copolymer containing 5 weight percent or less of ethylene and having a melt index of 6 to 20 or a polypropylene homopolymer having a melt index of 6 to 20.
One class polymer foams that can offer a variety of advantageous characteristics such as uniform cell size and structure, the appearance of solid plastic, etc. are microcellular foams. U.S. Pat. No. 4,473,665 (Martini-Vvedensky, et al.; Sep. 25, 1984) describes a process for making foamed polymer having cells less than about 100 microns in diameter. In the technique of Martini-Vvedensky, et al., a material precursor is saturated with a blowing agent, the material is placed under high pressure, and the pressure is rapidly dropped to nucleate the blowing agent and to allow the formation of cells. The material then is frozen rapidly to maintain a desired distribution of microcells.
U.S. Pat. No. 5,158,986 (Cha, et al.; Oct. 27, 1992) describes formation of microcellular polymeric material using a supercritical fluid as a blowing agent. In a batch process of Cha, et al., a plastic article is submerged at pressure in supercritical fluid for a period of time, and then quickly returned to ambient conditions creating a solubility change and nucleation. In a continuous process, a polymeric sheet is extruded, then run through rollers in a container of supercritical fluid at high pressure, and then exposed quickly to ambient conditions. In another continuous process, a supercritical fluid-saturated molten polymeric stream is established. The stream is rapidly heated, and the resulting thermodynamic instability (solubility change) creates sites of nucleation, while the system is maintained under pressure preventing significant growth of cells. The material then is injected into a mold cavity where pressure is reduced and cells are allowed to grow.
While polymer foams containing polypropylene exist, it would be advantageous, in terms of added simplicity and reduced cost, to be able to produce high-quality foams including polypropylene without the need for significant amounts of foam-controlability additives or other co-polymerized or blended polymer components. It is an object of the present invention to provide such articles.